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What does it matter just getting users to type su - username in the shell, if they must type their username and password in any way? Only 5 extra characters typing :).

If it's not acceptable by users and you are so lazy as not to want to write single script anymore, just install telnet and telnetd on the server and put the following alias in /etc/bash.bashrc.

alias x='telnet localhost'

Updated: Don't forget to restrict remote accesses to telnetd port (tcp/23). You can set up firewall (iptables) or traditional TCP wrapper (/etc/hosts.allow, /etc/hosts.deny - consult hosts_access(5) manual page).